An in-vehicle microphone apparatus mounted within a vehicle interior, for example, in a vehicle interior ceiling is conventionally widely known. A voice collected by the in-vehicle microphone apparatus is used for operation of in-vehicle devices (e.g., a car navigation apparatus) etc.
In a vehicle interior, various noise sounds are generated along with a user's speech sound (i.e., a target sound to be collected), including an engine sound, a sound of tires during running, and a wind sound. To accurately collect the target sound from such noise sounds, a directional microphone is often used for limiting a direction of picking up a sound (e.g., Japanese Laid-Open Patent Publication No. 2005-306278).
However, a conventional in-vehicle microphone has an unintended gap generated in the middle of a passage of sound going to a microphone element, and the target sound and the noise sounds cannot be separated in some cases. This will be described with reference to FIGS. 7 and 8.
FIG. 7 is a schematic configuration diagram of a conventional in-vehicle microphone apparatus 10 and FIG. 8 is a perspective view of an acoustic retainer 14 used in the in-vehicle microphone apparatus 10. This in-vehicle microphone apparatus 10 is assembled on a ceiling surface 100 of a vehicle interior and includes a microphone element 12, an acoustic retainer 14 retaining the microphone element 12, and a bezel 16 assembled on an end surface of the acoustic retainer 14. The microphone element 12 is disposed with a diaphragm 18 vibrating by receiving a sound and an electric circuit (not depicted) converting the vibration of the diaphragm into an electric signal. On the front side (the lower side on the plane of FIG. 7) of the diaphragm 18, a delay passage 20 is disposed that delays arrival of sound at the diaphragm 18.
The acoustic retainer 14 is provided with a first sound passage 22a guiding a sound to the back side of the diaphragm 18 and a second sound passage 22b guiding a sound to the front side of the diaphragm 18. End portions of the first sound passage 22a and the second sound passage 22b form a first sound collection port 24a and a second sound collection port 24b acting as inlet ports for sound. The first sound collection port 24a and the second sound collection port 24b are located on a directivity direction (target sound utterance direction) side and a side opposite to the directivity direction, respectively, in the end surface of the acoustic retainer 14. The bezel 16 is attached to the end surface of the acoustic retainer 14 to enhance the design of the in-vehicle microphone apparatus 10. The bezel 16 has a first through-hole 30a and a second through-hole 30b formed at positions directly facing the first sound collection port 24a and the second sound collection port 24b. 
A sound generated at a position opposite to the directivity direction, i.e., a portion constituting noise, goes through the first through-hole 30a, the first sound collection port 24a, and the first sound passage 22a and arrives at a back surface of the diaphragm 18, and another portion of the sound constituting noise goes through the second through-hole 30b, the second sound collection port 24b, the second sound passage 22b, and the delay passage 20 and arrives at a front surface of the diaphragm 18. The in-vehicle microphone apparatus 10 has sound path lengths adjusted such that the noise arrives at the front surface and the back surface of the diaphragm 18 at the same time. Since the noise arriving at the front surface and the back surface of the diaphragm 18 at the same time cancel out each other, the removal of the noise is achieved.
However, the bezel 16 of the conventional in-vehicle microphone apparatus 10 has a configuration coming into close contact only with a periphery of the acoustic retainer 14. Therefore, as depicted in FIG. 8, the conventional acoustic retainer 14 has a lip formed at the periphery for the close contact with the bezel 16. However, no lip exists that separates the first sound collection port 24a and the second sound collection port 24b. Thus, if the bezel 16 is brought into close contact with the acoustic retainer 14, a gap arises that allows the first sound collection port 24a and the second sound collection port 24b to communicate with each other between the acoustic retainer 14 and the bezel 16. As a result, this may cause a sound that should originally arrive at the back surface of the diaphragm 18 to go to the front surface of the diaphragm 18, or conversely cause a sound that should arrive at the front surface of the diaphragm 18 to go to the back surface of the diaphragm 18. Consequently, the sounds constituting noise cannot be properly canceled out and the target sound and the noise cannot clearly be separated.